Phenolic resole resins have long been cured with strong acids, such as sulfuric acid and organo-sulfonic acids. When used alone, these acids cause rapid hardening of most resole resins even at an ambient temperature, complicating the use of such resins for many applications particularly in making fiber reinforced plastic (FRP) and other resin composites and molded resin articles. The prior art, therefore, has long sought ways to retard the rate of resin cure at temperatures below the desired curing condition as a way of prolonging the pot life of such resin systems. To be useful, however, such cure rate retardation must be accomplished without degrading, to an unacceptable extent, the ultimate cure rate of the resin at the elevated cure temperature.
In U.S. Pat. No. 5,243,015, a latent catalyst comprising a salt of a primary or secondary amine and a strong acid is used. This latent catalyst improves storage stability (pot life) of the resole resin composition, while providing a cure rate at an elevated temperature comparable to the rate obtained using conventional strong acid catalysts. Strong acids also may be added to the composition, to an extent to act synergistically with the latent catalyst to accelerate the rate of reaction; but preferably not in an amount which would cure the resin at a commercially useful rate by itself.
U.S. Pat. No. 5,378,793 (European Patent Application 539,098) represents another approach. This patent discloses using a partial phosphate ester as a hardener for phenolic resole resins. The partial phosphate ester is prepared by reacting (partially esterifying) condensed phosphoric acids, such as orthophosphoric acid, pyrophosphoric acid, tetraphosphoric acid or phosphorus pentoxide under stringent conditions of temperature and vacuum, with polyols, such as glycerol, pentaertrythritol, sucrose and others to a constant free acidity value.
It is reported that the partial phosphate ester hardener exhibits a delayed action, i.e., the hardener cures the resole relatively slowly at a lower temperature, but allows a rapid cure when the temperature is increased. The patent attributes this result to dissociation of the ester at an elevated temperature to release free acid which acts as the hardener and promotes the cure reaction. At lower temperatures, however, the partial ester is stable (i.e., has a relatively low acid value) and does not contribute to a premature cure of the resole.
U.S. Pat. No. 5,864,003, incorporated herein in its entirety, represents another approach. This patent discloses using a latent curing agent selected from a particular class of nitrogen-containing acidic phosphorus compounds made from either phosphoric acid or phosphorous acid. Phosphoric acid equivalents may also be used such as pyrophosphoric acid, tetraphosphoric acid or phosphorus pentoxide as the phosphoric acid source.
Phenolic resole resins are cured in such applications as in making fiber reinforced plastic (FRP) and other resin composites and molded resin articles. The use of fiber reinforced plastic (FRP) and other resin composites and molded resin articles has experienced substantial growth as new applications of composites for products with greater strength and durability are devised. Many products are manufactured with FRP and other resin composites such as building materials, appliances, boats, or sporting equipment. For example, composites based on phenolic resins are applied in various products ranging from construction materials to mass transit trains, competing with and frequently replacing metals as the material of choice.
Studies have shown that FRP strengthened material such as phenolic resin composites possess fire resistance such that material strengthened with FRP can withstand sustained periods of time under fire conditions before failure would occur. In addition to fire resistance, phenolic resins under fire conditions emit low smoke or toxic fumes and resist the spread of flames. Such fire resistant qualities of the composites have increased interest and applicability of phenolic resin compositions. Phenolic resin compositions are thus becoming the material of choice for the production of composite parts, especially where public safety is critical.
Fabricators may utilize many production methods for making products with the composites. For example, in compression molding, a fabricator may compress compounds between heated matched mold surfaces to mold the compounds. Also, resin transfer molding (RTM) may be used in which a catalyzed resin is injected into matched molds and cured at particular temperatures.
However, composite fabrication processes providing cured phenolic resin compositions may generate undesirable voids in the resultant composite. The incidence of voids has been encountered using the latent curing agent of U.S. Pat. No. 5,864,003. In many applications of phenolic resins, voids must be minimized. Hence, there remains an interest in developing phenolic resin compositions having a cure behavior that provide a long pot life at temperatures below curing conditions, but exhibit a rapid hardening at elevated cure temperatures and result in composites with minimal voids.